While TrojanHorse showed that the executive lounge will be made available to Diamonds in the USA, the wording is different for Hiltons outside the USA (copied from the web site):
At Hilton hotels outside the U.S. with Executive Floors, Diamond VIPs will be provided with a guest room on the Executive Floor, whenever possible. In cases when there are no Executive Floors or where Executive Floor accommodations have been fully booked, Diamond VIP members will be offered a complimentary continental breakfast each morning or an alternative special amenity.
So, even though most are impressed by Hiltons outside the USA, it appears that they can deny Diamonds access to the lounge if the level is "full".